1. Field of the Invention
The present invention relates to the field of environmental scanning electron microscopes (ESEM) and more particularly to an apparatus and method of operating an ESEM with a wet sample.
2. Description of the Related Art
In a typical environmental scanning electron microscope (ESEM), an electron beam is emitted by an electron gun and passes through an electron optical column of an objective lens assembly. A series of pressure limiting apertures are placed along the column so as to maintain pressure differentials along the column. The pressure may vary from, for example, 10xe2x88x925 Pa at the electron gun up to 6000 Pa in the sample chamber through the use of a system of differential pumping. In the electron column, the electron beam passes through magnetic lenses which are used to focus the beam and direct the electron beam through a final pressure limiting aperture. The electron beam is then directed into the sample chamber wherein the beam impinges upon a sample.
The sample chamber is disposed below the vacuum column assembly and is capable of maintaining the sample enveloped in an imaging gas at a desired pressure. In addition to pressure, the temperature of the sample may also be controlled, typically through the use of a Peltier Device in the sample chamber.
The imaging gas may be altered to suit the sample under study and may be, for example carbon dioxide, nitrogen, air, water vapor or argon. Electron backscattering from the sample surface ionizes molecules of the imaging gas and thereby amplifies the signal to be detected. As water vapor is tolerated in the sample chamber, ESEM makes it possible to image wet samples. However, when imaging with water vapor, water evaporation or condensation on the sample should be considered. In some applications, for example, real time hydration or dehydration studies, condensation or dehydration may be desired and the temperature and pressure of the sample can be altered accordingly. However, for most applications, hydration and dehydration are not desired as it may change the sample and alter the corresponding image particularly when operating at low operating temperatures. When using water vapor as the imaging gas, at any pressure, there is a corresponding temperature which represents the saturated vapor pressure. The saturated vapor pressure of a liquid is the partial pressure of the vapor above its liquid state at equilibrium and is dependent on the type of liquid and its temperature.
This is illustrated in FIG. 1, which shows the general relationship between pressure and temperature for water vapor and the plotted line represents the saturated vapor pressure of water. If the pressure is raised or the temperature is lowered from this equilibrium, condensation would occur on the sample. Conversely, if the pressure is lowered or the temperature raised, evaporation would occur. Depending on the equipment used, it may be difficult and time consuming to adjust the pressure and temperature with the ESEM to obtain the saturated vapor pressure equilibrium. For example, control of either or both of the pressure and temperature may not be fine enough to easily obtain the saturated vapor pressure equilibrium particularly at low temperatures where the saturated vapor pressure of water is low. Accordingly, there remains a need in the art for efficiently imaging wet samples with an ESEM, and more particularly, efficiently imaging frozen samples with an ESEM.
An environmental scanning electron microscope is operated at an operating temperature and pressure within a sample chamber. A method of operating such an ESEM comprises:
a. placing a sample in the sample chamber;
b. humidifying a carrier gas such that the partial pressure of water vapor in the carrier gas corresponds to the vapor pressure of water at the operating temperature and the operating pressure; and
c. operating the environmental scanning electron microscope with the humidified carrier gas as the imaging gas.
The carrier gas may be any imaging gas, except for water vapor itself, such as, for example, carbon dioxide, air, nitrogen, argon or any combination thereof. The method may be used at low operating temperatures of the ESEM, such as, for example between 0xc2x0 C. and xe2x88x9240xc2x0 C. and even as low as xe2x88x92100xc2x0 C.
The carrier gas may be humidified by, for example, passing the carrier gas over a water reservoir. The water reservoir may be at a low temperature as low as xe2x88x9210xc2x0 C., xe2x88x9230xc2x0 C. or even xe2x88x92100xc2x0 C. and as high as 10xc2x0 C., 30xc2x0 C. or even 100xc2x0 C.
An imaging gas delivery system for an ESEM having a gas inlet comprises:
A. a carrier gas supply;
B. a gas humidifier coupled to the carrier gas supply;
C. a humidity sensor coupled to the gas humidifier; and
D. a delivery valve adapted to receive the gas inlet of the ESEM. When the system is in operation, a carrier gas circulates from the carrier gas supply, through the gas humidifier and humidity sensor to the gas inlet of the ESEM through the delivery valve. Thus the carrier gas becomes humidified and the humidified carrier gas can act as the imaging gas within the ESEM.
A circulation fan may be located between the gas supply and the gas humidifier to assist with flow of the carrier gas through the gas delivery system. Further, a controller may be connected to the system to control the degree of humidification of the carrier gas in response to the humidity sensor.
In an embodiment, the gas humidifier comprises a temperature regulated water reservoir. In such a gas humidifier, temperature control may be affected by, for example, a Peltier device. In another embodiment, the gas humidifier comprises a water injector such that water is directly added to the carrier gas.
These and other aspects of the invention will be evident upon reference to the attached figures and following detailed description.